(Not Applicable)
This invention relates in general to the application of ceramic coatings onto substrates, and in particular to methodology for controlling drying stress during shrinkage of a ceramic coating so applied through application of a drying control agent onto the outer surface of the coating after the coating is placed and minimally dried, and thereafter drying and heat-curing the coating.
Ceramic coatings are regularly applied to various substrates in a variety of product areas. One such product area is in aircraft construction, and includes such coatings on jet engine and engine exhaust components for thermal protection and energy absorption. One prior art method of coating placement involves the application of water-diluted, cement-based ceramic slurries by spraying or brushing the slurry to accomplish placement thereof, followed by ambient environment evaporation of the water of solution and final elevated-temperature curing to remove the water of hydration. The cements of the slurries typically are alkali metal silicates and bind together included ceramic oxides and energy absorbing fillers chosen for an intended application. The cement, or another compatible cement placed as a primer on the substrate, provides adhesion to the substrate.
Drying and curing a typical coating as described above results in volumetric reduction, i.e., shrinkage of the cement portion material and subsequent crack formation thereof. Cracking can also result from differences in thermal expansion of the substrate and the coating as well as from distortion or deformation of the substrate that may accompany heating and cooling thereof during a cure cycle. Once a crack penetrates the thickness of the coating, the crack then has a tendency to xe2x80x9ccurlxe2x80x9d or branch into cracks parallel to the coating-substrate interface. Such parallel cracks can result in an immediate loss of coating during cool-down from a cure temperature as well as during use where thermal cycling or high vibration occurs.
Generally, the characteristics of cracks in cement-based coatings relate to the total amount of shrinkage that occurs during the loss of the water of hydration, and additionally relies upon the coated surface area, the coating thickness, and the coating strength and condition at the time when cracking occurs. The amount of shrinkage is related to the water of solution initially in the slurry, the water of hydration that is removed during curing, and the strength of the material that is resisting shrinkage when the water is removed. Material that is weak when cracking begins may form many small cracks, while a strong material may form one or more large cracks. Because drying typically occurs from the surface into the thickness of the coating, cracks tend to initiate at the surface during drying and curing and propagate through the: thickness as the water diffuses to the surface for exit. Both large cracks in the coating and loss of coating are usual causes for rejection of coated or would-be coated products. Thus, desirable coatings have either no cracks or very narrow cracks, with the latter many times advantageous depending upon required local flexure of the coating in accord with characteristics of the coated substrate.
In view of present-day inefficiencies of typical ceramic coated substrates, it is evident that a need is present for a method of coating such substrates which produces a product with an intact coating. Accordingly, a primary object of the present invention is to provide methodology for controlling drying stresses during shrinkage of a ceramic coating on a substrate.
Another object of the present invention is to provide coating methodology wherein a moisture preservation agent is applied onto the outer surface layer of the applied coating prior to complete drying and curing thereof.
These and other objects of the present invention will become apparent throughout the description thereof which now follows.
The present invention is a method of controlling shrinkage and subsequent drying stresses of a ceramic coating coated onto a substrate. The method first includes applying a non-cured ceramic coating onto the substrate and permitting the coating to dry until a mechanically stable outer surface layer of the coating is formed. Once this stable outer surface layer is formed, a moisture control agent is generally uniformly applied onto the layer of the coating in a quantity sufficient to penetrate into the surface layer, with the result of such application being the inhibition of formation of a highly stressed dry outer skin as continued drying is permitted to occur. Once such drying is complete, the outer surface layer is subjected to an elevated temperature for curing and for evaporating therefrom the moisture preservation and drying control agent to thereby produce a cured-ceramic coated substrate whose coating is void of large cracks and whose durability is enhanced.
Operationally, the application of the moisture preservation agent prevents formation of a weak, dry crust or skin during ambient drying of the coating. In particular, the agent combines with suspension water in the outer layer of the coating to thereby displaces water at the surface, limit dehydration, and reduce the gradient in water content through the thickness of the coating. This reduced gradient evens out shrinkage throughout the thickness of the coating both during ambient-environment drying and early-stage elevated-temperature curing. During curing of the coating, and because of its outer surface-layer modification, the drying control agent acts to delay curing of the outer surface thereof to thereby ward off associated shrinkage and distributes drying stresses. This delay slows onset of cracking, provides more uniform curing throughout the thickness of the coating, and produces healing of any cracks that do form by causing a flow of material from the outer layer into the cracks. Continued elevated temperature exposure during the cure process evaporates the moisture preservation agent which ultimately results in allowing the cure of the outer layer. The resulting coating has minimal curling and has narrow cracks at most without any significant branching thereof parallel to the so-coated substrate, thus producing a coating with quality and durability during thermal reactivity.